Can 3 objects be replaced by 1 object, or joined by their location? (BGE)
The BGE does not support mesh manipulation that way. You can scale the objects (forming a longer Pipe) but you can't merge meshes.

When you let all objects listen to the same even (w) with the exact same behavior, they will behave exact the same way by ending themselves at the same time. This is what you told them to do.

What you wanted to do is to end the objects on the rollers. So you need to discriminate between objects on the rollers and objects that are not on the rollers. To be more specific you need to identify the three small pipes that can form the big pipe.

You could 


check what pipes are on the rollers. This excludes all other pipes.
check what orientation they have. This excludes misaligned pipes.
check the location. This excludes pipes too far away.


How to do?


You could assume that any pipe on a specific roller is already placed and aligned correctly 
Check all three rollers to have a pipe on top of itself 
or let the pipes check themselves if they are above a roller 


Checking options:


ray sensor (upward or downward)
collision sensor
radar sensor 
near sensor 
check via python